LIGA-process

The LIGA-process is used to manufacture micro structures by deep X-ray lithography. LIGA is the german acronym for lithography, electroplating and moulding (Lithographie, Galvanik und Abformung). The LIGA-process has been developed in the "Institut für Kernverfahrenstechnik" (IKVT), today "Institute for Microstructure Technology" (IMT) in the Forschungszentrum Karlsruhe GmbH since the late 1980th [Sai 2008]. Today several institutes use the LIGA-process, e.g. [Goe 2006].

Primarily the LIGA-process provides high aspect ratio micro structures in polymers like e.g. PMMA (better known as acrylic glass). Via electroplating these structures can be replicated in metals like gold, nickel, magnetic nickel-iron alloys or copper. Even replications in ceramics are possible. An industrial low cost production of micro structures is possible when a nickel tool is fabricated for hot embossing or injection moulding.

The main characteristics of LIGA-structures are:

large layout freedom in the mask geometry

high aspect ratios of up to >100 achievable

parallel side walls with flank angle very close to 90° (deviation: about 1 µm for 1 mm high structures)

First an intermediate X-ray absorption mask (IM) is made by electron beam writing a CAD generated layout into a resist layer (fig. 5). As substrate normally a silicon wafer is used, just because it is flat, smooth and not to expensive (fig. 1). This substrate is coated with a carbon layer in a sputter process. This layer is needed later to be able to separate the mask from the substrate. At the border of the wafer a few millimetres remain uncoated (fig. 2). Then the whole wafer is coated with a 2 µm thick titanium layer in another sputter process (fig. 3). The titanium will not stick to the carbon layer, but to the silicon wafer where the silicon is visible. This prevents the titanium layer to lift of from the wafer by internal stress. Titanium (low atomic number!) is used, because of its good transparency for X-rays. After this the wafer is spin coated with a 3.5 µm thick PMMA resist layer (fig. 4). PMMA is a positive resist. In the electron beam exposure step, in resist areas hit by the electrons, the long PMMA-molecules are damaged and can be dissolved in the following development step (fig. 6). In some cases also negative resists are used. These are cross-linked by the exposure, so exposed areas remain after the development process. Subsequently a 2.2 µm thick gold layer is deposited on the titanium layer where no resist covers the titanium (fig. 7). Gold (high atomic number!) is used because of its high X-ray absorption coefficient. In this step the thickness of the gold is critical: when the gold is overgrowing the resist layer, the wanted geometry of the mask is lost and it becomes useless. In the next step the remaining resist is dissolved in a solvent suitable for non-exposed PMMA (fig. 8). Then a 6 mm thick invar-steel (mechanically invariant to temperature changes) frame is glued onto the titanium membrane, giving mechanical stability to the fragile membrane (fig. 9). With a knife the titanium membrane is cut through at the outer contour of the frame (fig. 10) and the mask (fig. 12) is carefully separated from the substrate (fig. 11). The X-ray absorption contrast of the intermediate mask is sufficient to produce structures of up to about 70 µm thickness by X-ray lithography. As in most cases this height will not be sufficient, a so called working mask with a higher X-ray absorption contrast is needed.

2. Making a working mask (WM):

A working mask is an X-ray lithographic copy of an intermediate mask with the aim to get a mask with a higher X-ray absorption contrast. One way to make a working mask starts with a steel plate with a polished front side (fig. 13). The back side of this plate is thinned by milling, leaving only a wall of a few millimetres (fig. 14). The front side is coated with a 2 µm titanium membrane by sputtering (fig. 15). Then a 60 µm thick PMMA resist layer is deposited on the titanium membrane (fig. 16). The resist layer is structured by X-ray lithography via the intermediate mask (fig. 17). After developing the exposed areas (fig. 18) 25 µm thick gold absorbers are electroplated (fig. 19). Subsequently the remaining resist is exposed to X-rays without using a mask, so it can be stripped afterwards in the same developer (fig. 20). The last step is to etch the cavity in the back side of the mask with a selective etching medium, so the steel is removed and only the free standing titanium membrane is remaining (fig. 21).

In the deep X-ray lithography step a shadow projection of the working mask into a quite thick PMMA resist layer (100 µm to 3 mm) is performed (fig. 22, 23). PMMA layers of this thickness mostly are glued to the substrate. Normally a proximity distance of 50 µm to 150 µm is introduced between the mask and the resist surface to protect the mask from damages that might result from a contact between mask and resist. The direct lithographic micro structure is finished after a development step (fig. 24).

4. Electro forming:

When metal micro structures are required, an electro forming step is added. In this case the micro structures are fabricated on an electrically conducting substrate or the substrate and the micro structures are covered with a thin layer of e.g. gold. Then, in the electro forming step the required metal is deposited (fig. 25). When the process is stopped before the metal layer is higher than the resist structure, the produced structures can be used as metallic micro structures. As the original resist micro structures still remain on the substrate, a flood exposure step (without mask) is introduced to be able to dissolve the resist structures afterwards (fig. 26). The metallic micro structures have to be separated from the substrate (fig. 27). This can be done by mechanical force when the surface of the substrate is very smooth, so the adhesion between the structures and the substrate is low. If the forces become to high, the micro structures might be deformed. In this case a sacrificial layer (e.g. titanium) on the substrate can be used. This layer is etched selectively (e.g. with HF), so the metallic micro structures are separated from the substrate (fig. 28).

When a mass production is required, a nickel mould has to be fabricated. In this case the electro forming step is continued until the deposited nickel layer has reached several millimetres in thickness (fig. 29, 30). Then this nickel block (fig. 31) is removed from the substrate (e.g. an 8 mm thick copper block) and by wire-cut electrical discharge machining it is brought into a precisely defined outer shape (fig. 32, 33). Again a flood exposure (fig. 34) is used to prepare the removing of the remaining resist in the mould (fig. 35).

5. Mass replication by hot embossing or injection moulding:

The nickel moulding tool can then be used several ten thousands of cycles to replicate micro structures via hot embossing or injection moulding (fig. 36). In the case of hot embossing the resulting structures mostly have to be machine finished (fig. 37).